This invention relates to rubber modified styrene polymers. More particularly this invention relates to the use of conjugated diene butyl rubber elastomers as novel rubber modifiers for preparing high impact strength styrene and styrene-acrylonitrle copolymer based thermoplastic resins.
Impact resistant rubber modified polystyrene and styrene-acrylonitrile (hereafter SAN) copolymers are well known in the art and are commercially available. In such products, polybutadiene rubber or styrene butadiene rubber are used almost exclusively as the elastomeric modifiers at levels of rubber content of about 3 to 15% by weight but some materials containing up to about 30% elastomers are also available.
In both rubber modified polystyrene and rubber modified SAN polymers, the latter being generally known as the ABS resins, the elastomeric component is dispersed into the styrene or SAN copolymer, the system being two-phase in nature.
These materials are conventionally prepared by the mass, mass suspension or emulsion polymerization processes. In the mass and mass suspension processes the rubber polymer is dissolved in styrene monomer and the monomer is then polymerized. The mass suspension process uses a dispersion of the polymers in water in the final stages of the polymerization. In the emulsion polymerization technique monomer and rubber latices are combined and the monomer is graft polymerized, this method usually being employed with rubber modification of styrene-acrylonitrile thermoplastics. Low unsaturation rubbers such as butyl rubber and EPDM (as defined in ASTM D-1418-72A) have been found generally ineffective as impact strength modifiers for styrene and SAN polymers, a sufficient degree of grafting and rubber crosslinking not being achieved. The present invention is based on the discovery that conjugated diene butyl rubber elastomers exhibit high reactivity and may be successfully incorporated into styrene or SAN polymers using conventional processes to prepare products having good impact strength properties.
In accordance with the present invention, there have been discovered elastomer modified polystyrene or SAN thermoplastics, the elastomer being a conjugated diene elastomer copolymer consisting of from 85 to 99.5% by weight of a C.sub.4 -C.sub.7 isoolefin combined with 15 to 0.5% by weight of a conjugated diolefin having 4 to 14 carbon atoms, the elastomer being further characterized as containing randomly distributed sites of conjugated diene unsaturation. A more specific embodiment comprises compositions wherein the conjugated diene elastomer is produced by dehydrohalogenating a halogenated isobutylene-isoprene butyl rubber to produce a copolymer having randomly inserted conjugated diene unsaturation in the isoprene units of said copolymer.
A further embodiment of the present invention constitutes blends of said CDB elastomers with chlorinated butyl rubber as suitable rubber modifiers for styrene or SAN thermoplastics. While chlorinated butyl rubber alone is not considered suitable, it has been found in accordance with the present invention that chlorinated butyl rubber may be employed in combination with the conjugated diene butyl rubber elastomers in blends containing 90% to 10% by weight of chlorinated butyl rubber. Such blends containing 60-80% by weight chlorinated butyl rubber and 40 to 20% by weight of CDB elastomer are particularly suitable.
The CDB rubber modified compositions of the present invention contain the elastomeric component in what are considered conventional amounts for high impact strength styrene or SAN plastics, that is, about 3 to 10% by weight, or more, such as up to about 15% for rubber modified SAN thermoplastics, of rubber modifier being present in the finished product. Higher concentrations such as up to about 30 or 40% may also be used if such formulations are desired. A particular advantage of the present invention is that no change in processing is required, the CDB or CDB-chlorinated butyl blends being adaptable to the usual methods for making impact resistant styrene or SAN polymers.
The compositions of the present invention meet the various structural and morphological requirements for the rubber phase in rubber modified styrene or SAN systems: a discrete rubber phase exists with particle size 1 to 10 microns, a graft layer of polymerized styrene or styrene-acrylonitrile is formed on the surface of the rubber particle and controlled crosslinking of the rubber modifier is present.
The compositions of this invention are prepared by conventional mass and emulsion polymerization techniques wherein the elastomer is graft polymerized with styrene or styrene-acrylonitrile. Both grafting and crosslinking are necessary in order to achieve the desired impact strength properties of the rubber modified plastic composition.
The conjugated diene butyl elastomeric copolymers useful in the present invention are known and are described in U.S. Pat. No. 3,816,371 issued June 11, 1974 to Baldwin et al, incorporated herein by reference.
These conjugated diene butyl elastomers may be generally described as copolymers consisting of from 85 to 99.5% by weight of a C.sub.4 -C.sub.7 isoolefin combined with 15 to 0.5% by weight of a conjugated diolefin having 4 to 14 carbon atoms, the copolymer containing randomly distributed conjugated diene unsaturation. Moreover, as described in U.S. application Ser. No. 465,479 filed by Baldwin et al on Apr. 30, 1974, the conjugated diene functionality may be present such that both olefin units are in the backbone chain, or both are outside of the chain, or one may be inside the chain and one outside, or both may be present in a ring, such as by dehydrohalogenation of allylic halogen of cyclopentadiene. Dehydrohalogenation of a butyl-type polymer prepared from dimethylbutadiene and isobutylene can provide conjugated olefinic structures having the following configurations: ##STR1##
Preferably the copolymers useful in the present invention are copolymers of isobutylene and isoprene, a major portion of the isoprene units combined therein having conjugated diene unsaturation and may be represented by the following structure: ##STR2## where n+m represent the number of isoolefin units incorporated into the butyl rubber polymer backbone and m represents the number of conjugated diolefin units present substantially as isolated units.
The preferred method of preparing the elastomers useful in the present invention is through dehydrohalogenation of a halogenated butyl rubber.
The term "butyl rubber" is used in the industry to describe copolymers made from a reaction mixture containing 70 to 99.5% by weight of a C.sub.4 -C.sub.7 isoolefin such as isobutylene and 30 to b 0.5% by weight of a C.sub.4 -C.sub.14 conjugated multiolefin such as isoprene. The resulting copolymers contain 85 to 99.5% by weight of isoolefin and 0.5 to 15% of combined multiolefin. The polymer backbone of commercial butyl rubber consists primarily of isobutylene units with just a few percent of isoprene units, the latter contributing the small amount of unsaturation present in butyl rubber.
Halogenated butyl rubber, such as chlorinated butyl rubber, is described in U.S. Pat. No. 3,099,644 and is typified by the following formula (X being a halogen, chlorine or bromine): ##STR3## Halogenated butyl rubber may be prepared by halogenating butyl rubber in a solution containing 1 to 60% by weight butyl rubber in a substantially inert C.sub.5 -C.sub.8 hydrocarbon solvent such as pentane, hexane, heptane and contacting the solution with halogen gas for about 2 to 25 minutes whereby halogenated copolymers form containing up to 1 or somewhat more halogen atom per double bond initially present. Illustrative is "Chlorobutyl 1068" a chlorinated butyl rubber (isobutylene-isoprene) which before halogenation contains about 1.8 mole % unsaturation and a viscosity average molecular weight of about 450,000. Chlorinated butyl rubber generally will contain about 0.5 to 3.0 wt. % of combined chlorine, preferably 1-2 wt. %.
The conjugated diene butyl elastomers useful in the present invention are preferably prepared via dehydrohalogenation of a halogenated rubber butyl rubber. Generally this process comprises contacting a solution of halogenated butyl rubber with: (1) a soluble metal carboxylate where the metal is selected from the metal of Groups Ib, IIb, IVa and VIII of the Periodic Table; (2) a soluble carboxylic acid; and (3) an oxide or a hydroxide of the metal selected from Groups Ia and IIa of the Periodic Table. This dehydrohalogenation process is described in detail in U.S. Pat. No. 3,775,387 issued Nov. 27, 1973 to Baldwin et al and is incorporated herein by reference.
The mole percent of conjugated diene (CD) unsaturation in the elastomers useful in the present invention may generally be described as being from about 0.5 to about 2.5 and these conjugated diene butyl elastomer copolymers have a number average molecular weight generally from about 5,000 to 500,000. While preferred levels of CD functionality are set forth hereinbelow, no absolute levels of CD functionality have been established since CDB reactivity in preparing the products of the present invention depends on not only the degree of conjugated diene functionality but also the structure of the elastomer with respect to the location of the conjugated unsaturation. As is recognized in the art, unsaturation predominantly outside the main polymer chain structures being shown hereinabove, gives products with a much greater degree of reactivity.
The styrene and SAN polymers useful in the present invention are well known in the art and only brief mention need be made thereof. Unmodified general purpose polystyrene is a high molecular weight (Mw=2-3.times.10.sup.5) crystal clear thermoplastic having an Izod impact value of 0.2 ft.-lb./in. and a specific gravity of about 1.05. Unmodified SAN copolymers generally contain about 24% acrylonitrile and have Izod impact values of about 0.4 (Izod impact values reported herein were determined in accordance with ASTM D-256-73 on samples molded at 450.degree. F.).
In the present invention it has further been found that the level of conjugated diene functionality in the elastomer can control the rubber phase particle size and the extent of grafting and crosslinking of the dispersed rubber phase when styrene is modified with a CDB elastomer using the mass process. The mole % of conjugated diene (CD) units is preferably less than about 1.1 mole % CD average. Such an elastomer can be obtained from a CDB prepared to contain 1.1% or less mole % conjugated diene, or by blending 3 parts by weight of a 0.66 mole % CD CDB elastomer and 1 part of a 1.3 mole % CD CDB elastomer, or by blending 3 parts of a chlorinated butyl rubber, such as a chlorinated isobutylene-isoprene copolymer, which contains no conjugated diene units, with 1 part of a 1.3 mole % CD CDB elastomer. The most desirable physical properties have been obtained with these blends and conjugated diene proportions. For optimum results, such as ease of adaptability of the CDB polymers to conventional processes, the minimum amount of conjugated diene functionality present should be about 0.7 mole %.
On the other hand, when SAN copolymers as opposed to styrene alone are modified with CDB latices in accordance with the present invention, a relatively greater mole % of conjugated diene functionality can be employed. In such materials, for example, the use of about 1.3 mole % conjugated diene CDB elastomers at levels of about 6 to 15% by weight based on the combined weight of elastomer and SAN gives products with desirable properties.
The invention is further illustrated by the following examples which are not to be considered as limitative of its scope. All blends reported are parts by weight, e.g., 3/1 means 3 parts blended with 1 part by weight. In the examples the following test procedures were used.
Izod Impact: ASTM D-256-73, notched specimens 1/2.times.1/8.times.5 in., molded at 450.degree. F.
Tensile Properties: ASTM D-638-72, specimen 1/4.times.1/8.times.11/4 in. (Type IV), 0.2 in./min. strain rate
Melt Index: ASTM D-1238-62T, 3 gram sample, Condition G.
(Properties reported usually reflect a minimum of ten Izod specimens, three tensile specimens and three melt index measurements).
CDB elastomers used in the examples were conjugated diene isobutylene-isoprene copolymers prepared by dehydrohalogenation of chlorinated butyl rubber.